On my '94 XJ12 the idle is very fast, 1200 rpm or more,
the car will go down the road @ 30mph without your foot on the gas.

How can I slow this down?
I did a "search" but was unable to find a answer.

I'm not a mechanic, so please use simple terms ;-)
Pictures & hyroglifics are greatly appreciated.

Thanks in Advance
Jeff

 

Hi Jeff,

I'm not an expert on the 12-cylinder engines, but off the top of my head here are a few possible causes of high idle:

1. Throttle body butterfly valve not closing fully, possibly due to accumulated "gunk."

2. Air leak in the air intake plumbing somewhere between the Mass Air Flow sensor (MAF) and the cylinder head (intake manifold gasket) allowing unmetered air into the system. Unmetered air is not measured by the MAF, so the Engine Control Module (ECM) cannot adjust fueling for the excess air because it doesn't know about it.

3. The Extra Air Valve may be stuck open. It is Part 6 in the diagram below and may be mounted on the intake manifold with a couple of rubber hoses attached to it. It's part of the Idle Control system:



4. Weak throttle or throttle cable tension spring(s) not closing throttle. One clue that this might be a problem is if the accelerator pedal doesn't come all the way up when your foot is released.

5. Faulty Engine Coolant Temperature sensor telling ECM coolant is cold even after it has warmed up, causing the ECM to continue to apply cold-start fuel enrichment.

6. Bent throttle connecting rod.

Hopefully others with have additional ideas.

Cheers,

Don

 

Hey Don,
Thanks for the advise and the diagram
I'll try it this weekend.
Jeff

 

I had the same problem but 1500rpm
There are numerous vacuum pipes that come off the inlet manifold and dependant on which country the car is specified for some of them are blanked off. The rubber blanks perish and fall off letting extra air into the inlet manifold raising the rpm. The pipes also go to the heater control system. Try squashing the pipes that are connected correctly and if the tickover reduces there may be a pipe off out of sight in the central console area. There are also pipes to the air filter boxes, my air con man found one of these off. I had at least two pipes off and the extra air valve was sticking. I took this out and ran it through the dishwasher which cleaned the valve part and cycling it in cups of boiling water with a few drops of thin oil solved the problem. After doing all this the engine wanted to tick over at 400rpm which is too low and it will not keep running at such a low speed. You can then increase tickover by adjusting the throttle linkages. 750 is the correct setting I believe but I have set to run at 650 and it's fine at that and is easier to hold on the brake at junctions.
Kind regards
Charlie

 

Thanks
Jeff